1. Field of the Invention
The present invention relates to a thermal transfer sheet utilizing a sublimative dye, and more specifically to a black thermal transfer sheet which permits to form an image having a high density in black color and a high color rendering property, and excellent in fastness against heat and light.
2. Description of the Related Art
A gradation image or a monotone image such as characters, symbols or the like has conventionally been formed on an image receiving sheet by means of a thermal transfer printing method. With respect to such a thermal transfer printing method, there have widely been applied a sublimation type thermal transfer printing method and a heat-fusion type thermal transfer printing method.
Of these printing methods, according to the sublimation type thermal transfer printing method, an image is formed by placing, on an image receiving sheet, a thermal transfer sheet in which a dye layer obtained by dissolving or dispersing a sublimative dye used as a coloring agent into a binder made of resin, has previously been carried on a substrate film, and impressing energy corresponding to image information by means of a heating means such as a thermal head or a laser, to cause the dye contained in the sublimative dye layer on the thermal transfer sheet to transfer into the image receiving sheet, thereby forming the image.
According to the above-described sublimation type thermal transfer printing method, it is possible to control a transferred amount of dye in a unit of a single dot by changing an amount of energy to be impressed to the thermal transfer sheet, with the result that there can be formed an image with gradation in full colors, thus leading to realization of an image having a high quality which is equal to an image obtained by a silver film. Public attention has therefore been attracted by such a sublimation type thermal transfer printing method, and this method has been utilized as an information recording means in many kinds of field of art.
Along with the development of various hardware and software having a relationship to multimedia, the market of products for the sublimation type thermal transfer printing method has been enlarged in a hard copy system in full color such as a static image obtained by a computer-graphics, satellite communications or the like, a digital image obtained by an exemplary CD-ROM or the like, and an analog image obtained by a video recorder.
There are many actual uses of the image receiving sheet utilizing the sublimation type thermal transfer printing method. There can be mentioned exemplary uses such as output of proofs or images for printing; output of a blueprint and a design with the use of a CAD, a CAM or the like; output for many kinds of medical analyzing or measuring equipment such as a CT scanner, a camera for an endoscope or the like; output of a photograph of a person's face of cards such as an identification card, a credit card or the like, and output of a composite photograph or a commemorative photograph in an amusement park, a video arcade, a museum, an aquarium or the like.
In the thermal transfer sheet utilizing the above-described sublimation type thermal transfer printing method, a color reproducibility, in particular, a black color reproducibility with a high density is required as an important feature. More specifically, when an attempt is made to perform the reproduction in full color with the use of three primary colors of yellow, magenta and cyan, a dye layer exclusive to the black color is required, because the color reproducibility of jet-black color is poor due to restrictions in the subtractive process, the thermal transfer technique or the like.
With respect to a black thermal transfer sheet having a dye layer exclusive to the black color, it has been known to mix the dyes of yellow, magenta and cyan to obtain a black dye layer. This method is disclosed in for example, Japanese Patent Provisional Publication No. S61-258,791, No. H1-1136,787, No. H3-205,188, No. H4-267,197 and No. H7-232,481.
However, absorption spectrum of the black color obtained by mixing three primary colors of yellow, magenta and cyan has absorption wave troughs in the vicinity of 490 nm and 580 nm, thus causing a problem that the black color may visually be perceived in different color by an illumination light having a different wavelength. In general, this phenomenon is called "color rendering".
In the field of fiber dyeing, it is known to add an orange dye or a violet dye other than three primary colors, in order to improve the color rendering property.
However, in the field of the thermal transfer recording, a vigorous development in a dye of intermediate color has not as yet been made, and there has not been found out a dye of intermediate color having an excellent fastness against heat and light, and an excellent stability in an image receiving sheet after the completion of the thermal printing.
In addition, when a dye having a different hue was mixed, there has often been recognized a phenomenon of occurrence of intense discoloration by means of light. In general, this phenomenon is called "catalytic discoloration by light". The black dye layer contains a dye having a different due mixed therein, and accordingly, there is a problem that the catalytic discoloration by light tends to easily occur.